1. Field of the Invention
The present invention relates to a device for controlling the air-fuel ratio of an internal combustion engine, which feeds, to the internal combustion engine, the fuel in an amount that meets the operation condition of the engine by using a signal detected by an air-fuel ratio sensor. More specifically, the invention relates to a device for controlling the air-fuel ratio of an internal combustion engine, which is capable of highly precisely finding learning correction values in an open-loop operation region without executing the air-fuel ratio feedback control operation, by using an air-fuel ratio sensor constituted by an ordinary oxygen sensor.
2. Prior Art
In a device for controlling the air-fuel ratio of an internal combustion engine, in general, a target air-fuel ratio is set depending upon operation condition data from various sensors (air-flow sensor that measures the amount of the air taken in, etc.), and the amount of fuel injection is so corrected that the practical air-fuel ratio comes into agreement with the target air-fuel ratio (usually, stoichiometric air-fuel ratio xcex=14.7).
In the device for controlling the air-fuel ratio of an internal combustion engine, further, an air-fuel ratio sensor (also called xe2x80x9coxygen sensorxe2x80x9d) is usually disposed in the exhaust pipe to detect the stoichiometric air-fuel ratio while learning and correcting the air-fuel ratio feedback control quantity in order to compensate for a change caused by aging and dispersion of various parts constituting the sensors and the fuel-feeding system.
FIG. 5 is a diagram schematically illustrating the constitution of a conventional device for controlling the air-fuel ratio of an internal combustion engine.
In FIG. 5, an intake pipe 2 of an engine 1 constituting the main body of the internal combustion engine is provided with a throttle valve 3 for adjusting the amount of the air taken in, and a throttle opening-degree sensor 4 for measuring the opening degree xcex8 of the throttle valve 3 is coupled to the throttle valve 3.
An air-flow sensor 5 is provided on the upstream side of the throttle valve 3 in the intake pipe 2, and an injector 6 is provided in the intake pipe 2 on the downstream side of the throttle valve 3 to inject fuel in a required amount.
The air-flow sensor 5 measures the flow rate of the air in the intake pipe 2 as the intake air amount Qa taken in by the engine 1.
A combustion chamber 7 in each cylinder of the engine 1 is constituted by a cylinder block 8 and a piston 9 that reciprocates in the cylinder block.
The combustion chamber 7 is provided with a spark plug 10, an intake valve and an exhaust valve 12.
The combustion chamber 7 is connected to an exhaust pipe 13. An air-fuel-ratio sensor 14 which is an oxygen sensor is disposed in the exhaust pipe 13.
The air-fuel-ratio sensor 14 produces an air-fuel ratio corresponding to the stoichiometric air-fuel ratio xcex.
Data (throttle opening degree xcex8, intake air amount Qa, air-fuel ratio signal AF) detected by the sensors 4, 5 and 14 and representing the operation conditions of the engine 1 are input to a control circuit 20 which is a microcomputer.
Though not diagramed, the control circuit 20 includes a well-known CPU, RAM and ROM connected to the CPU through a bidirectional bus, as well as input ports and output ports.
The control circuit 20 includes air-fuel ratio correction means for correcting the air-fuel ratio so as to accomplish a target air-fuel ratio depending upon the operation conditions, feedback control condition-determining means for determining the conditions for controlling the feedback of air-fuel ratio to the engine 1 depending upon the operation conditions, feedback control means for bringing the air-fuel ratio of the engine 1 into agreement with the target air-fuel ratio when the control conditions are permitted, and air-fuel ratio learning means for learning and correcting the air-fuel ratio feedback control quantity. The control circuit 20 controls the amount of fuel injected through the injector 6 based upon the operation conditions and the air-fuel ratio signal AF.
To the input ports of the control circuit 20 are connected the throttle opening-degree sensor 4, air-flow sensor 5, air-fuel ratio sensor 14, as well as various other sensors (rotation sensor for detecting the rotational speed of the engine, cooling water temperature sensor, etc.) that are not shown.
The control circuit 20 processes various input data (operation conditions) to obtain control data of the engine 1, and produces, through the output ports thereof, injection signals J for the injectors 6, ignition signals G for the spark plugs 10, as well as drive signals for various other actuators that are not shown.
Next, the operation of the conventional device for controlling the air-fuel ratio of an internal combustion engine shown in FIG. 5 will be concretely described with reference to FIG. 6.
FIG. 6 is a diagram schematically illustrating learning correction values obtained by using a conventional device for controlling the air-fuel ratio of an internal combustion engine disclosed in Japanese Examined Patent Publication (Kokoku) No. 56340/1987.
FIG. 6 illustrates learning correction values ZC0 to ZC9 in plural operation regions to where the air-fuel ratio feedback control is applied, and in which the abscissa represents the engine rotational speed Ne [r/min], the ordinate represents filling efficiency EC [%] corresponding to the intake air amount Qa, i.e., represents the engine load.
The feedback operation regions are sectionalized by the engine rotational speeds NC0 to NC2 and the engine loads EC0 to EC2.
The learning correction values ZC0 to ZC9 in the operation regions of FIG. 6 are obtained by sampling the air-fuel ratio feedback control quantities among the predetermined ignition cycles, and are periodically updated at every update timing when the sampling is finished.
In FIG. 5, first, the control circuit 20 operates the target air-fuel ratio and the target ignition timing based on the operation condition data from various sensors, and produces injection signals J for the injectors 6 and ignition signals G for the spark plugs 10.
Therefore, the injector 6 is driven just before the intake stroke of the engine 1 to inject fuel, whereby the mixture gas containing fuel is taken into the combustion chamber 7 when the throttle valve 3 is opened, so that the interior of the combustion chamber is uniformly filled with the mixture gas.
The spark plug 10 is energized near the compression stroke of the engine 1 to ignite the mixture gas in the combustion chamber 7, whereby the engine 1 produces a drive torque as a result of combustion.
On the other hand, feedback control means in the control circuit 20 executes the air-fuel feedback control operation when the condition for controlling the air-fuel ratio feedback is established depending upon the operation conditions of the engine 1.
At this moment, the feedback control means operates the control quantity based upon the operation conditions and the air-fuel ratio signal AF from the air-fuel ratio sensor 14, and so controls the feedback that the practical air-fuel ratio is brought into agreement with the target air-fuel ratio.
Thus, the air-fuel ratio of the engine 1 is controlled to accomplish a target value, whereby the catalytic converter (not shown) for purifying the exhaust gases disposed in the exhaust pipe 13 purifies the exhaust gases to a sufficient degree preventing the emission of non-purified gases.
Further, the amount of controlling the fuel (air-fuel ratio) is corrected not only by the air-fuel ratio signals AF but also by the learning correction values ZC0 to ZC9 in the operation regions of FIG. 6 depending upon the operation regions, whereby the air-fuel ratio of the engine 1 is highly precisely controlled to acquire a target air-fuel ratio.
When it is desired to obtain a large output torque for rapid acceleration or to obtain cooling effect in a high-speed operation region, on the other hand, the amount of fuel is increased to enrich the air-fuel ratio of the engine 1 (to render the air-fuel ratio to be smaller than the stoichiometric air-fuel ratio). Therefore, feedback control for changing the stoichiometric air-fuel ratio xcex to the target value is inhibited, and the open-loop operation is carried out.
In the open-loop operation region, the learning correction values are not operated. Therefore, the correction control operation is executed by using learning correction values (ZC3, ZC6, ZC7 to ZC9, etc. in FIG. 6) in the feedback (closed-loop) operation region close to the open-loop operation region.
However, since they are not the learning correction values in the practical open-loop operation region, it is not allowed to highly precisely control the air-fuel ratio in the open-loop operation region.
Further, in a conventional device disclosed in, for example, Japanese Examined Patent Publication (Kokoku) No. 56499/1990, the EGR inhibition region is set in the feedback operation region close to the open-loop operation region, and the learning correction values fed back in the EGR inhibition region are used in the open-loop operation region.
In this case, too, however, the operation conditions of the engine 1 are different from those of the practical open-loop operation region, and the air-fuel ratio cannot be highly precisely controlled.
In the conventional device for controlling the air-fuel ratio of an internal combustion engine as described above, the learning correction values in the feedback (closed-loop) operation region are used as learning correction values in the open-loop operation region, involving a problem in that a highly precisely enriched air-fuel ratio cannot be obtained due to dispersion in the engine 1 and in various control equipment.
Besides, when the air-fuel ratio being controlled is enriched to an excess degree due to dispersion in the air-fuel ratio in the open-loop operation region, properties of the exhaust gases are deteriorated. Conversely, when the air-fuel ratio is not enriched to a sufficient degree, the catalytic converter is damaged.
Further, the cost is driven up when it is attempted to use, for example, a linear air-fuel ratio sensor in order to highly precisely control the enriched air-fuel ratio in the open-loop operation region.
The present invention was accomplished in order to solve the above-mentioned problems, and its object is to provide a device for controlling the air-fuel ratio of an internal combustion engine, which is capable of highly precisely finding the learning correction values in the open-loop operation region by using an air-fuel-ratio sensor which is an ordinary oxygen sensor.
The present invention is concerned with a device for controlling the air-fuel ratio of an internal combustion engine comprising:
an injector for injecting the fuel of a required amount into an internal combustion engine;
an air-fuel-ratio sensor which is an oxygen sensor installed in an exhaust pipe of the internal combustion engine for detecting the stoichiometric air-fuel ratio; and
a control circuit for controlling the injector based upon the operation conditions of the internal combustion engine and the air-fuel-ratio signal from the air-fuel-ratio sensor;
wherein the control circuit includes:
air-fuel-ratio correction means for correcting the amount of fuel depending upon a target air-fuel ratio that varies according to the operation conditions;
feedback control condition-determining means for determining the conditions for controlling the air-fuel ratio feedback of the internal combustion engine depending upon the operation conditions;
feedback control means for so controlling the air-fuel ratio of the internal combustion engine as to come into agreement with the target air-fuel ratio in the feedback operation region where the control conditions are permitted; and
air-fuel-ratio learning means for finding learning correction values for every operation region based on the control quantity of the feedback control means;
wherein in the open-loop operation region where the control conditions are not permitted, the feedback control means temporarily executes a feedback control operation, and the air-fuel-ratio learning means finds learning correction values in the open-loop operation region during feedback control period that lasts only temporary.
In the device for controlling the air-fuel ratio of an internal combustion engine according to the invention, the air-fuel-ratio learning means changes the number of times of sampling for finding the learning correction values depending upon the feedback operation region and the open-loop operation region.
In the device for controlling the air-fuel ratio of an internal combustion engine according to the invention, the air-fuel-ratio learning means sets the number of times of sampling for finding the learning correction values in the open-loop operation region to be smaller than the number of times of sampling for finding the learning correction values in the feedback operation region.
In the device for controlling the air-fuel ratio of an internal combustion engine according to the invention, the feedback control means changes the proportional coefficient and integration coefficient of the control quantity depending upon the feedback operation region and the open-loop operation region.
In the device for controlling the air-fuel ratio of an internal combustion engine according to the invention, the feedback control means sets the proportional coefficient and the integration coefficient in the open-loop operation region to be larger than the proportional coefficient and the integration coefficient in the feedback operation region.
In the device for controlling the air-fuel ratio of an internal combustion engine according to the invention, the air-fuel-ratio learning means has filter operation means for executing the filtering every time when the learning correction value is updated.
In the device for controlling the air-fuel ratio of an internal combustion engine according to the invention, the air-fuel ratio learning means changes the filtering coefficient of the filter operation means depending upon the feedback operation region and the open-loop operation region.
In the device for controlling the air-fuel ratio of an internal combustion engine according to the invention, the air-fuel-ratio learning means sets the filter coefficient in the open-loop operation region to be larger than the filter coefficient in the feedback operation region.
In the device for controlling the air-fuel ratio of an internal combustion engine according to the invention, the air-fuel-ratio learning means divides the open-loop operation region into plural regions depending upon the rotational speed and load of the internal combustion engine, and sets the learning correction values for the divided regions.